As an extrusion molding die for use in manufacturing a metallic hollow extrusion molded product, such as, e.g., an aluminum heat exchanging tube for use in car air-conditioning heat exchangers, there are a die called a porthole die as shown in FIG. 18A, a die called a spider die as shown in FIG. 18B, or a die called a bridge die as shown in FIG. 18C.
In these extrusion molding dies, a male die 1 and a female die 2 are combined so that the mandrel 1a of the male die 1 is disposed in the corresponding die hole 2a of the female die 2 to define a circular extrusion hole by and between the mandrel 1a and the die hole 2a. In the die, it is configured such that a metal billet (metal material) pressed against the billet pressure receiving surface (metal material pressure receiving surface 1b) of the male die 1 is introduced in both the dies 1 and 2 via material introduction holes 1c and then passes through the extrusion hole while being plastically deformed, so that an extrusion molded article having a cross-section corresponding to the cross-sectional configuration of the extrusion hole is formed.
In such extrusion molding dies, large stress due to pressing of the metal billet is applied to the billet pressure receiving surface 1b of the male die 1, causing generation of cracks in the periphery of the pressure receiving portion of the die by the stress, which may sometimes make it difficult to attain sufficiently long die life.
Under the circumstances, conventionally, extrusion molding dies for metal material as disclosed by the below-listed Patent Documents 1 and 2 have been conventionally proposed. In the dies, it is configured such that the billet pressure receiving surface of the male die is formed into a convex configuration protruded in a direction opposite to the billet extruding direction (i.e., protruded rearward) so that the pressing force of the metal billet to be applied to the billet pressure receiving surface can be received by a bridge portion of the male die.    Patent Document 1: Japanese Unexamined Laid-open Utility Model Publication No. S53-102938 (see claims, FIGS. 3 to 5)    Patent Document 2: Japanese Examined Laid-open Patent Publication No. H06-81644 (see claims, and drawings)